Known stereo camera systems comprise two single-image cameras, each of which preferably comprise a CCD sensor and a fixed focal length lens system or, alternatively, a CMOS image sensor and such a lens system. Preferably, the single-image cameras simultaneously take one image each. These images are referred to as an image pair. With the aid of the images taken, a direct distance measurement of objects visible in both images can be made. For determining the distance, it is important to precisely know, in addition to the camera parameters, also the orientation of the optical axes of the single-image cameras with respect to one another. A stable construction can for example be chosen, by means of which the two cameras only slightly change their position to one another under intended operating conditions. As a result thereof, stability over an extended period of time is achieved for a calibrated system. In addition to the relatively complex structure, which also occupies a considerable amount of space, considerable inaccuracy of the measurement result can occur during distance measurement as a result of changes in position of the cameras with respect to one another despite the stable construction. This requires a range limit for the distance measurement in order to obtain measurement results within a reasonable tolerance. Only by means of a stable construction and a limitation of the measurement accuracy and the range, can distance measurements usefully be made over a longer period of time with such a stereo camera system without complex calibration services having to be performed in the meantime.
If, however, a higher accuracy and/or the determination of greater distances is required, a readjustment of the stereo camera system or a re-calibration has to be carried out. It is possible to achieve a higher measurement accuracy by way of a perfect mechanical orientation of the optical axes of the camera system, or to determine the actual positions of the optical axes for example by the determination of error angles and to take these actual positions into account as calibration parameters in the evaluation of the recorded images. These positions can in particular be taken into account in the evaluation algorithm for determining the distance of an object. One criterion for the practicability of a stereo camera system is that during normal operation no servicing is required for adjustment and calibration and that disturbances are recognized and removed as automatically as possible.
The calibration of known stereo camera systems having the stable mechanical construction described is carried out with the aid of special master images, of which one image each is taken with the aid of the single-image cameras. In order to ensure a high measurement accuracy of the stereo camera system during distance measurement, this calibration has to be repeated at time intervals which are dependent on the environmental conditions. During the calibration process, in the images recorded by the single-image cameras of the stereo camera system prominent corresponding two-dimensional object images are measured. Usually, from at least nine such corresponding object images a so called fundamental matrix is determined using a reliable method. This fundamental matrix contains both a rotation matrix of the two cameras with respect to one another and their distance vector. However, given this method the position determination of the image contents with the aid of the cameras is only possible with an accuracy that depends on the camera resolution. Thus, an ultra-precise calibration of the camera system is not possible with the aid of such a known method.
Further, if there is a relative movement between the stereo camera system and an observed object, another known method for self-calibration of the stereo camera system can be used. With the aid of an image sequence recorded by one of the cameras of the stereo camera system, the relationship of the movements of the object traced over several images with respect to its total distance can be derived, i.e. can be calculated. As a result of the measurement of the same object in the image pairs of the two cameras of the stereo camera system, a faulty distance curve is determined. Subsequently, the two measured values are compared to one another, as a result whereof the error angles of the calibration can be estimated. In this known method, too, the accuracy with which the error angles can be determined is dependent on the accuracy of the position determination of the objects with the aid of the object images recorded by the cameras.